We spend around a third of our lives (around 25 years) sleeping; it is vital to our survival, but despite years of research, scientists still aren't entirely sure why we do it. The urge to sleep is all-consuming, and if we are deprived of it, we will eventually slip into slumber even if the situation is life threatening.

Sleep is common to mammals, birds and reptiles and has been conserved through evolution, even though it prevents us from performing other useful tasks, such as eating, reproducing and raising young. It is as important as food for keeping us alive; without it, rats will die within two or three weeks – the same amount of time that it takes to die from starvation.

Sleep can be divided into two broad stages, non-rapid eye movement (NREM), and rapid eye movement (REM) sleep. The vast majority of our sleep (around 75 to 80 per cent) is NREM, characterised by electrical patterns in the brain known as ‘sleep spindles’ and high, slow delta waves. This is the time we sleep the deepest.

Without NREM sleep, our ability to form declarative memories, like learning to associate pairs of words, is seriously impaired; deep sleep is important for transferring short-term memories into long-term storage. Deep sleep is also the time of peak growth hormone release in the body, which is important for cell reproduction and repair.

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The purpose of REM sleep is unclear; the effects of REM sleep deprivation are less severe than NREM deprivation, and for the first two weeks humans report little in the way of ill effects.

REM sleep is the period during the night when we have our most vivid dreams, but people dream during both NREM and REM sleep. During NREM sleep, dreams tend to be more concept-based, whereas during REM sleep dreams are more vivid and emotional.

Some scientists argue that REM sleep allows our brains a safe place to practice dealing with situations or emotions that we might not encounter during our daily lives; during REM sleep our muscles are temporarily paralysed, preventing us acting out these emotions. Others think that it might be a way to unlearn memories, or to process unwanted feelings or emotions. Each of these ideas has its flaws, and no one knows the real answer.

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During the night, you cycle through five separate stages of sleep every 90 to 110 minutes, experiencing between three and five dream periods each night.

The five stages of sleep can be distinguished by changes in the electrical activity in your brain, measured by electroencephalogram (EEG). The first stage begins with drowsiness as you drift in and out of consciousness, and is followed by light sleep and then by two stages of deep sleep. Your brain activity starts to slow down, your breathing, heart rate and temperature drop, and you become progressively more difficult to wake up. Finally, your brain perks up again, resuming activity that looks much more like wakefulness, and you enter rapid eye movement (REM) sleep; the time that your most vivid dreams occur. This cycle happens several times throughout the night, and each time, the period of REM sleep grows longer.

Stage 1 (1-7 minutes)

During the first stage of sleep you are just drifting off; your eyelids are heavy and your head starts to drop. During this drowsy period, you are easily awoken and your brain is still quite active. The electrical activity on an electroencephalogram (EEG) monitor starts to slow down, and the cortical waves become taller and spikier. As the sleep cycle repeats during the night, you re-enter this drowsy half-awake, half-asleep stage.

Stage 2 (10-25 minutes)

After a few minutes, your brain activity slows further, and you descend into light sleep. On the EEG monitor, this stage is characterised by further slowing in the waves with an increase in their size, and short one or two-second bursts of activity known as ‘sleep spindles’. By the time you are in the second phase of sleep, your eyes stop moving, but you are still woken easily.

Stage 3 (20-40 minutes)

As you start to enter this stage, your sleep spindles stop, showing that your brain has entered moderate sleep. This is then followed by deep sleep. The trace on the EEG slows still further as your brain produces delta waves with occasional spikes of smaller faster waves in between. As you progress through stage-three sleep, you become much more difficult to wake up.

Stage 4 (20-40 minutes)

There is some debate as to whether sleep stages three and four are really separate, or whether they are part of the same phase of sleep. Stage four is the deepest stage, and during this time, you are extremely hard to wake. The EEG shows tall, slow waves known as delta waves, your muscles relax and your breathing becomes slow and rhythmic, which can lead to snoring.

Stage 5 (10-60 minutes)

After deep sleep, your brain starts to perk up, and its electrical activity starts to resemble the brain when it is awake. This is the period of the night when most dreams happen. Your muscles are temporarily paralysed, and your eyes dart back and forth, giving this stage its name, rapid eye movement (REM) sleep.

Jodie Tyley is editor of How It Works Magazine. The latest issue is out now: Issue 77 ‘The Power of Magnetism’, Subscribe for your copy here